Primary objectives of knee arthroplasty include restoration of limb alignment, ligament balance, and joint surface contours. Restoration of limb alignment in extension reduces asymmetric loading of the bicondylar design, probably reducing the risk of component loosening. Restoration of ligament balance and joint surface contours prevents instability and restores normal joint kinematics.
Surgical instrumentation serves to simplify and standardize the implantation procedure. Several operative techniques have been advocated, differing in instrumentation and in the sequence of bone cuts. The present invention may be usefully employed in any of the several techniques.
A total knee joint prosthesis typically includes a tibial platform and a femoral component having condyler portions adapted to rest in complementary surfaces of the tibial component. Restoration of ligament balance requires careful positioning of these tibial and femoral components and selective lengthening of contracted ligamentous or soft-tissue structures. Ideally, the ligaments should be balanced at every position of knee flexion. To simplify the surgical technique, the ligament balance is checked mainly at full knee extension and at 90.degree. knee flexion.
Extension Balancing
In full extension, both limb alignment and ligament balance must be restored. The distal femoral bone cut is made perpendicular to the axis between the centers of the hip and knee joints. The proximal tibial cut is made perpendicular to the longitudinal axis of the tibial shaft. Contracted ligaments and soft tissues are then lengthened by partial release as needed to equalize their length and tension in extension. After these bone and soft tissue cuts are made, the gap between distal femoral surface and proximal tibial surface (the extension gap) should accommodate the combined thickness of the tibial and femoral components, with the medial and lateral knee ligaments equally tensioned to provide stability.
Historically, ligament balance in extension has been addressed by the use of either spacer blocks or a joint distraction instrument. Spacer blocks are oval, metallic shims of the approximate shape and size of the proximal tibial platform. After proximal tibial and distal femoral cuts are made, spacer blocks of increasing thickness are placed in the extension gap. Gentle medial (valgus) and lateral (varus) stresses are applied to the knee by the surgeon, and the relative lengths and tensions of medial and lateral ligaments are assessed. Incremental releases are performed as needed to balance the ligaments.
In the alternative technique, a joint distraction instrument is placed between the distal femoral condyles and the previously cut proximal tibial surface during full knee extension. When the force between each condyle and the tibia is equal, the ligaments are equally tensioned. Partial ligament releases are then performed while maintaining equal ligament tension. When the proper ligament alignment is restored, the distal femoral cut is made parallel to the proximal tibial cut.
Flexion Balancing
In 90.degree. knee flexion, ligament balance can be restored by changing the alignment of the posterior femoral bone cut. The tibial cut and soft tissue releases also affect the flexion gap, but these factors are predetermined by the requirements of limb alignment and ligament balance in extension. Historically, the posterior femoral bone cut has been positioned by posterior condyle referencing or by the use of a joint distraction instrument.
Posterior condyle referencing involves empirically positioning the posterior femoral cut to remove slightly more medial than lateral femoral condyle. This external rotation of the cut corrects the most common clinical situation, where the flexion gap is tight medially. However, the posterior condyle referencing technique becomes difficult when the condyles are eroded or when a severe pre-operative deformity exists. Technical errors are possible which may cause malfunction of the implanted knee prosthesis.
A joint distraction instrument places a distracting force between each of the posterior femoral condyles and the previously cut tibial surface during 90.degree. knee flexion. If the force between each condyle and the tibia is equal, the ligaments will be nearly equally tensioned. If the extension gap has already been balanced, the position of the posterior femoral cut can then be measured from the proximal tibial surface to create a flexion gap of the appropriate size and shape. The joint distraction technique for flexion gap balancing is appealing in theory, but joint distraction instruments in the prior art are difficult to use. Because of shortcomings of such instrumentation, many surgeons use the easier but less reliable posterior condyle referencing technique for positioning the posterior femoral bone cut.
U.S. Pat. No. 5,116,338 to Poggie (1992) describes a flexion gap distractor which fits into the space between posterior condyles and tibial plateau. The device suffers from a number of disadvantages. It must be forced into a tight space between the posterior condyles and the tibial plateau. The portions of the device which articulate with the posterior femoral condyles fit poorly with very large and very small to knees. Medial and lateral tension adjustments are separate and tactile feed-back of tension is poor, making it difficult to achieve equal tension in the medial and lateral soft-tissues. The size of the device obstructs visualization of the knee joint and obstructs access to the anterior tibial plateau, preventing the use of spacer blocks to position the posterior femoral cut.
U.S. Pat. No. 4,938,762 to Wehrli (1990) describes a flexion gap distractor which attaches to the tibial shaft with a bone screw and engages the femoral intercondylar notch with a bent metallic arm. This device also suffers from a number of disadvantages. The need for a screw in an uninvolved and remote portion of the tibia creates undesirable morbidity. The distraction device is free to rotate around the tibial bone screw and therefore does not control medial to lateral motion of the femur relative to the tibia. Such medial or lateral motion can alter the soft-tissue balance in a deceptive manner. Finally, no means is provided for measuring the space between proximal tibial cut and the proposed posterior femoral cut.